1. Field of the Invention
This invention relates to polyisocyanate compositions and is more particularly concerned with storage stable liquid polyisocyanate compositions containing a release agent formed in situ, and with a method for the preparation of said compositions.
2. Description of the Prior Art
The use is known of organic polyisocyanates as binders, or as components of a binder, for the preparation of particle boards; see, for example, U.S. Pat. Nos. 3,428,592; 3,440,189; 3,557,263; 3,636,199; 3,870,665; 3,919,017 and 3,930,110.
In a typical process the binder resins, optionally in the form of a solution or aqueous suspension or emulsion, are applied to or admixed with particles of cellulosic or like material capable of being compressed into particle board, using a tumbler apparatus or blender or other form of agitator. The mixture of particles and binder is then formed into a mat and subjected to heat and pressure using heated platens. The process can be carried out in a batch operation or continuously. To avoid adhesion of the board so formed to the heated platens it has hitherto been necessary to interpose a sheet, impermeable to isocyanate, between the surface of the board and the platen during the forming process, or to coat the surface of the platen, prior to each molding operation, with an appropriate release agent or to coat the surface of the particles themselves with a material which will not adhere to the platen. Any of these alternatives, particularly where the process is being operated on a continuous basis, is cumbersome and a drawback to what is otherwise a very satisfactory method of making a particle board with highly attractive structural strength properties.
It has recently been found that the above drawbacks to the use of organic isocyanates as particle board binders can be overcome in a very satisfactory manner by incorporating certain phosphorus-containing compounds as internal release agents. These findings are disclosed in copending application Ser. No. 35,647 filed May 3, 1979. Among the phosphorus-containing compounds which can be employed in accordance with the above findings are pyrophosphates derived from mono- and di-hydrocarbyl acid phosphates and related compounds. In accordance with the process described in the aforesaid pending application Ser. No. 35,647 the said pyrophosphates can be prepared separately by reacting the corresponding acid phosphates with dehydrating agents such as phosgene, phosphorus oxychloride and the like and, if desired, can then be mixed with the polyisocyanate for use in the formation of the particle boards. Alternatively, it is also disclosed in the aforesaid pending application that said pyrophosphates can be prepared by reaction with the dehydrating agent in the presence of the organic polyisocyanate.
We have now found that, by using very carefully controlled conditions, it is possible to prepare said pyrophosphates and related polyphosphates in situ in said organic polyisocyanates without the use of additional reagents such as the ecologically and toxicologically undesirable compounds phosgene, phosphorus oxychloride, and the like. We have further found that it is possible to prepare organic polyisocyanates which contain said pyrophosphates and related polyphosphates produced in situ and which possess the additional desirable characteristics of being liquid and storage-stable, i.e. they show no tendency to deposit solids or to separate into two liquid phases on storage for prolonged periods of time. This is in direct contrast to the behaviour of compositions made in a closely related manner but not observing the specific reaction conditions which are discussed in detail hereafter.
The ability to produce such liquid, storage-stable, polyisocyanate compositions greatly facilitates the utilization of polyisocyanates as particle board binders. Thus, it is possible thereby to provide the particle board manufacturer with a single composition, prepared in a highly economical manner, which can be utilized readily by the manufacturer in accordance with the process of the aforesaid Ser. No. 35,647. The polyisocyanate compositions prepared in accordance with this invention can be applied, either as such or as an aqueous emulsion or suspension, to the cellulosic or like particles which are subsequently molded, using heat and pressure, into particle boards. The latter, by reason of the pyrophosphate produced in situ in the compositions of the invention, release readily from the mold platens.